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Versions: 00 01 02 03 04 05 rfc5629                                     
SIPPING                                                     J. Rosenberg
Internet-Draft                                               dynamicsoft
Expires: August 16, 2004                               February 16, 2004


   A Framework for Application Interaction in the Session Initiation
                             Protocol (SIP)
            draft-ietf-sipping-app-interaction-framework-01

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at http://
   www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 16, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.

Abstract

   This document describes a framework for the interaction between users
   and Session Initiation Protocol (SIP) based applications. By
   interacting with applications, users can guide the way in which they
   operate. The focus of this framework is stimulus signaling, which
   allows a user agent to interact with an application without knowledge
   of the semantics of that application. Stimulus signaling can occur to
   a user interface running locally with the client, or to a remote user
   interface, through media streams. Stimulus signaling encompasses a
   wide range of mechanisms, ranging from clicking on hyperlinks, to
   pressing buttons, to traditional Dual Tone Multi Frequency (DTMF)
   input. In all cases, stimulus signaling is supported through the use
   of markup languages, which play a key role in this framework.



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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.    Definitions  . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.    A Model for Application Interaction  . . . . . . . . . . . .  7
   3.1   Functional vs. Stimulus  . . . . . . . . . . . . . . . . . .  8
   3.2   Real-Time vs. Non-Real Time  . . . . . . . . . . . . . . . .  9
   3.3   Client-Local vs. Client-Remote . . . . . . . . . . . . . . .  9
   3.4   Presentation Capable vs. Presentation Free . . . . . . . . . 10
   3.5   Interaction Scenarios on Telephones  . . . . . . . . . . . . 11
   3.5.1 Client Remote  . . . . . . . . . . . . . . . . . . . . . . . 11
   3.5.2 Client Local . . . . . . . . . . . . . . . . . . . . . . . . 11
   3.5.3 Flip-Flop  . . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.    Framework Overview . . . . . . . . . . . . . . . . . . . . . 13
   5.    Application Behavior . . . . . . . . . . . . . . . . . . . . 16
   5.1   Client Local Interfaces  . . . . . . . . . . . . . . . . . . 16
   5.1.1 Discovering Capabilities . . . . . . . . . . . . . . . . . . 16
   5.1.2 Pushing an Initial Interface Component . . . . . . . . . . . 16
   5.1.3 Updating an Interface Component  . . . . . . . . . . . . . . 18
   5.1.4 Terminating an Interface Component . . . . . . . . . . . . . 18
   5.2   Client Remote Interfaces . . . . . . . . . . . . . . . . . . 19
   5.2.1 Originating and Terminating Applications . . . . . . . . . . 19
   5.2.2 Intermediary Applications  . . . . . . . . . . . . . . . . . 19
   6.    User Agent Behavior  . . . . . . . . . . . . . . . . . . . . 21
   6.1   Advertising Capabilities . . . . . . . . . . . . . . . . . . 21
   6.2   Receiving User Interface Components  . . . . . . . . . . . . 21
   6.3   Mapping User Input to User Interface Components  . . . . . . 23
   6.4   Receiving Updates to User Interface Components . . . . . . . 23
   6.5   Terminating a User Interface Component . . . . . . . . . . . 23
   7.    Inter-Application Feature Interaction  . . . . . . . . . . . 25
   7.1   Client Local UI  . . . . . . . . . . . . . . . . . . . . . . 25
   7.2   Client-Remote UI . . . . . . . . . . . . . . . . . . . . . . 26
   8.    Intra Application Feature Interaction  . . . . . . . . . . . 27
   9.    Example Call Flow  . . . . . . . . . . . . . . . . . . . . . 28
   10.   Security Considerations  . . . . . . . . . . . . . . . . . . 33
   11.   Contributors . . . . . . . . . . . . . . . . . . . . . . . . 34
         Normative References . . . . . . . . . . . . . . . . . . . . 35
         Informative References . . . . . . . . . . . . . . . . . . . 36
         Author's Address . . . . . . . . . . . . . . . . . . . . . . 36
         Intellectual Property and Copyright Statements . . . . . . . 37











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1. Introduction

   The Session Initiation Protocol (SIP) [1] provides the ability for
   users to initiate, manage, and terminate communications sessions.
   Frequently, these sessions will involve a SIP application. A SIP
   application is defined as a program running on a SIP-based element
   (such as a proxy or user agent) that provides some value-added
   function to a user or system administrator. Examples of SIP
   applications include pre-paid calling card calls, conferencing, and
   presence-based [10] call routing.

   In order for most applications to properly function, they need input
   from the user to guide their operation. As an example, a pre-paid
   calling card application requires the user to input their calling
   card number, their PIN code, and the destination number they wish to
   reach. The process by which a user provides input to an application
   is called "application interaction".

   Application interaction can be either functional or stimulus.
   Functional interaction requires the user agent to understand the
   semantics of the application, whereas stimulus interaction does not.
   Stimulus signaling allows for applications to be built without
   requiring modifications to the client. Stimulus interaction is the
   subject of this framework. The framework provides a model for how
   users interact with applications through user interfaces, and how
   user interfaces and applications can be distributed throughout a
   network. This model is then used to describe how applications can
   instantiate and manage user interfaces.























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2. Definitions

   SIP Application: A SIP application is defined as a program running on
      a SIP-based element (such as a proxy or user agent) that provides
      some value-added function to a user or system administrator.
      Examples of SIP applications include pre-paid calling card calls,
      conferencing, and presence-based [10] call routing.

   Application Interaction: The process by which a user provides input
      to an application.

   Real-Time Application Interaction: Application interaction that takes
      place while an application instance is executing. For example,
      when a user enters their PIN number into a pre-paid calling card
      application, this is real-time application interaction.

   Non-Real Time Application Interaction: Application interaction that
      takes place asynchronously with the execution of the application.
      Generally, non-real time application interaction is accomplished
      through provisioning.

   Functional Application Interaction: Application interaction is
      functional when the user device has an understanding of the
      semantics of the interaction with the application.

   Stimulus Application Interaction: Application interaction is
      considered to be stimulus when the user device has no
      understanding of the semantics of the interaction with the
      application.

   User Interface (UI): The user interface provides the user with
      context in order to make decisions about what they want. The user
      enters information into the user interface. The user interface
      interprets the information, and passes it to the application.

   User Interface Component: A piece of user interface which operates
      independently of other pieces of the user interface. For example,
      a user might have two separate web interfaces to a pre-paid
      calling card application - one for hanging up and making another
      call, and another for entering the username and PIN.

   User Device: The software or hardware system that the user directly
      interacts with in order to communicate with the application. An
      example of a user device is a telephone. Another example is a PC
      with a web browser.






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   User Input: The "raw" information passed from a user to a user
      interface. Examples of user input include a spoken word or a click
      on a hyperlink.

   Client-Local User Interface: A user interface which is co-resident
      with the user device.

   Client Remote User Interface: A user interface which executes
      remotely from the user device. In this case, a standardized
      interface is needed between the user device and the user
      interface. Typically, this is done through media sessions - audio,
      video, or application sharing.

   Media Interaction: A means of separating a user and a user interface
      by connecting them with media streams.

   Interactive Voice Response (IVR): An IVR is a type of user interface
      that allows users to speak commands to the application, and hear
      responses to those commands prompting for more information.

   Prompt-and-Collect: The basic primitive of an IVR user interface. The
      user is presented with a voice option, and the user speaks their
      choice.

   Barge-In: In an IVR user interface, a user is prompted to enter some
      information. With some prompts, the user may enter the requested
      information before the prompt completes. In that case, the prompt
      ceases. The act of entering the information before completion of
      the prompt is referred to as barge-in.

   Focus: A user interface component has focus when user input is
      provided fed to it, as opposed to any other user interface
      components. This is not to be confused with the term focus within
      the SIP conferencing framework, which refers to the center user
      agent in a conference [12].

   Focus Determination: The process by which the user device determines
      which user interface component will receive the user input.

   Focusless User Interface: A user interface which has no ability to
      perform focus determination. An example of a focusless user
      interface is a keypad on a telephone.

   Presentation Capable UI: A user interface which can prompt the user
      with input, collect results, and then prompt the user with new
      information based on those results.





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   Presentation Free UI: A user interface which cannot prompt the user
      with information.

   Feature Interaction: A class of problems which result when multiple
      applications or application components are trying to provide
      services to a user at the same time.

   Inter-Application Feature Interaction: Feature interactions that
      occur between applications.

   DTMF: Dual-Tone Multi-Frequency. DTMF refer to a class of tones
      generated by circuit switched telephony devices when the user
      presses a key on the keypad. As a result, DTMF and keypad input
      are often used synonymously, when in fact one of them (DTMF) is
      merely a means of conveying the other (the keypad input) to a
      client-remote user interface (the switch, for example).

   Application Instance: A single execution path of a SIP application.

   Originating Application: A SIP application which acts as a UAC,
      calling the user.

   Terminating Application: A SIP application which acts as a UAS,
      answering a call generated by a user. IVR applications are
      terminating applications.

   Intermediary Application: A SIP application which is neither the
      caller or callee, but rather, a third party involved in a call.























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3. A Model for Application Interaction


         +---+            +---+            +---+             +---+
         |   |            |   |            |   |             |   |
         |   |            | U |            | U |             | A |
         |   |   Input    | s |   Input    | s |   Results   | p |
         |   | ---------> | e | ---------> | e | ----------> | p |
         | U |            | r |            | r |             | l |
         | s |            |   |            |   |             | i |
         | e |            | D |            | I |             | c |
         | r |   Output   | e |   Output   | f |   Update    | a |
         |   | <--------- | v | <--------- | a | <.......... | t |
         |   |            | i |            | c |             | i |
         |   |            | c |            | e |             | o |
         |   |            | e |            |   |             | n |
         |   |            |   |            |   |             |   |
         +---+            +---+            +---+             +---+

               Figure 1: Model for Real-Time Interactions

   Figure 1 presents a general model for how users interact with
   applications. Generally, users interact with a user interface through
   a user device. A user device can be a telephone, or it can be a PC
   with a web browser. Its role is to pass the user input from the user,
   to the user interface. The user interface provides the user with
   context in order to make decisions about what they want. The user
   enters information into the user interface. The user interface
   interprets the information, and passes it to the application. The
   application may be able to modify the user interface based on this
   information. Whether or not this is possible depends on the type of
   user interface.

   User interfaces are fundamentally about rendering and interpretation.
   Rendering refers to the way in which the user is provided context.
   This can be through hyperlinks, images, sounds, videos, text, and so
   on. Interpretation refers to the way in which the user interface
   takes the "raw" data provided by the user, and returns the result to
   the application in a meaningful format, abstracted from the
   particulars of the user interface. As an example, consider a pre-paid
   calling card application. The user interface worries about details
   such as what prompt the user is provided, whether the voice is male
   or female, and so on. It is concerned with recognizing the speech
   that the user provides, in order to obtain the desired information.
   In this case, the desired information is the calling card number, the
   PIN code, and the destination number. The application needs that
   data, and it doesn't matter to the application whether it was
   collected using a male prompt or a female one.



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   User interfaces generally have real-time requirements towards the
   user. That is, when a user interacts with the user interface, the
   user interface needs to react quickly, and that change needs to be
   propagated to the user right away. However, the interface between the
   user interface and the application need not be that fast. Faster is
   better, but the user interface itself can frequently compensate for
   long latencies there. In the case of a pre-paid calling card
   application, when the user is prompted to enter their PIN, the prompt
   should generally stop immediately once the first digit of the PIN is
   entered. This is referred to as barge-in. After the user-interface
   collects the rest of the PIN, it can tell the user to "please wait
   while processing". The PIN can then be gradually transmitted to the
   application. In this example, the user interface has compensated for
   a slow UI to application interface by asking the user to wait.

   The separation between user interface and application is absolutely
   fundamental to the entire framework provided in this document. Its
   importance cannot be overstated.

   With this basic model, we can begin to taxonomize the types of
   systems that can be built.

3.1 Functional vs. Stimulus

   The first way to taxonomize the system is to consider the interface
   between the UI and the application. There are two fundamentally
   different models for this interface. In a functional interface, the
   user interface has detailed knowledge about the application, and is,
   in fact, specific to the application. The interface between the two
   components is through a functional protocol, capable of representing
   the semantics which can be exposed through the user interface.
   Because the user interface has knowledge of the application, it can
   be optimally designed for that application. As a result, functional
   user interfaces are almost always the most user friendly, the
   fastest, the and the most responsive. However, in order to allow
   interoperability between user devices and applications, the details
   of the functional protocols need to be specified in standards. This
   slows down innovation and limits the scope of applications that can
   be built.

   An alternative is a stimulus interface. In a stimulus interface, the
   user interface is generic, totally ignorant of the details of the
   application. Indeed, the application may pass instructions to the
   user interface describing how it should operate. The user interface
   translates user input into "stimulus" - which are data understood
   only by the application, and not by the user interface. Because they
   are generic, and because they require communications with the
   application in order to change the way in which they render



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   information to the user, stimulus user interfaces are usually slower,
   less user friendly, and less responsive than a functional
   counterpart. However, they allow for substantial innovation in
   applications, since no standardization activity is needed to build a
   new application, as long as it can interact with the user within the
   confines of the user interface mechanism. The web is an example of a
   stimulus user interface to applications.

   In SIP systems, functional interfaces are provided by extending the
   SIP protocol to provide the needed functionality. For example, the
   SIP caller preferences specification [13] provides a functional
   interface that allows a user to request applications to route the
   call to specific types of user agents. Functional interfaces are
   important, but are not the subject of this framework. The primary
   goal of this framework is to address the role of stimulus interfaces
   to SIP applications.

3.2 Real-Time vs. Non-Real Time

   Application interaction systems can also be real-time or
   non-real-time. Non-real interaction allows the user to enter
   information about application operation in asynchronously with its
   invocation. Frequently, this is done through provisioning systems. As
   an example, a user can set up the forwarding number for a
   call-forward on no-answer application using a web page. Real-time
   interaction requires the user to interact with the application at the
   time of its invocation.

3.3 Client-Local vs. Client-Remote

   Another axis in the taxonomization is whether the user interface is
   co-resident with the user device (which we refer to as a client-local
   user interface), or the user interface runs in a host separated from
   the client (which we refer to as a client-remote user interface). In
   a client-remote user interface, there exists some kind of protocol
   between the client device and the UI that allows the client to
   interact with the user interface over a network.

   The most important way to separate the UI and the client device is
   through media interaction. In media interaction, the interface
   between the user and the user interface is through media - audio,
   video, messaging, and so on. This is the classic mode of operation
   for VoiceXML [3], where the user interface (also referred to as the
   voice browser) runs on a platform in the network. Users communicate
   with the voice browser through the telephone network (or using a SIP
   session). The voice browser interacts with the application using HTTP
   to convey the information collected from the user.




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   In the case of a client-local user interface, the user interface runs
   co-located with the user device. The interface between them is
   through the software that interprets the users input and passes them
   to the user interface. The classic example of this is the web. In the
   web, the user interface is a web browser, and the interface is
   defined by the HTML document that it's rendering. The user interacts
   directly with the user interface running in the browser. The results
   of that user interface are sent to the application (running on the
   web server) using HTTP.

   It is important to note that whether or not the user interface is
   local, or remote (in the case of media interaction), is not a
   property of the modality of the interface, but rather a property of
   the system. As an example, it is possible for a web-based user
   interface to be provided with a client-remote user interface. In such
   a scenario, video and application sharing media sessions can be used
   between the user and the user interface. The user interface, still
   guided by HTML, now runs "in the network", remote from the client.
   Similarly, a VoiceXML document can be interpreted locally by a client
   device, with no media streams at all. Indeed, the VoiceXML document
   can be rendered using text, rather than media, with no impact on the
   interface between the user interface and the application.

   It is also important to note that systems can be hybrid. In a hybrid
   user interface, some aspects of it (usually those associated with a
   particular modality) run locally, and others run remotely.

3.4 Presentation Capable vs. Presentation Free

   A user interface can be capable of presenting information to the user
   (a presentation capable UI), or it can be capable only of collecting
   user input (a presentation free UI). These are very different types
   of user interfaces. A presentation capable UI can provide the user
   with feedback after every input, providing the context for collecting
   the next input. As a result, presentation capable user interfaces
   require an update to the information provided to the user after each
   input. The web is a classic example of this. After every input (i.e.,
   a click), the browser provides the input to the application and
   fetches the next page to render. In a presentation free user
   interface, this is not the case. Since the user is not provided with
   feedback, these user interfaces tend to merely collect information as
   its entered, and pass it to the application.

   Another difference is that a presentation-free user interface cannot
   support the concept of a focus. As a result, if multiple applications
   wish to gather input from the user, there is no way for the user to
   select which application the input is destined for. The input
   provided to applications through presentation-free user interfaces is



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   more of a broadcast or notification operation, as a result.

3.5 Interaction Scenarios on Telephones

   This same model can apply to a telephone. In a traditional telephone,
   the user interface consists of a 12-key keypad, a speaker, and a
   microphone. Indeed, from here forward, the term "telephone" is used
   to represent any device that meets, at a minimum, the characteristics
   described in the previous sentence. Circuit-switched telephony
   applications are almost universally client-remote user interfaces. In
   the Public Switched Telephone Network (PSTN), there is usually a
   circuit interface between the user and the user interface. The user
   input from the keypad is conveyed used Dual-Tone Multi-Frequency
   (DTMF), and the microphone input as Pulse Code Modulated (PCM)
   encoded voice.

   In an IP-based system, there is more variability in how the system
   can be instantiated. Both client-remote and client-local user
   interfaces to a telephone can be provided.

   In this framework, a PSTN gateway can be considered a "user proxy".
   It is a proxy for the user because it can provide, to a user
   interface on an IP network, input taken from a user on a circuit
   switched telephone. The gateway may be able to run a client-local
   user interface, just as an IP telephone might.

3.5.1 Client Remote

   The most obvious instantiation is the "classic" circuit-switched
   telephony model. In that model, the user interface runs remotely from
   the client. The interface between the user and the user interface is
   through media, set up by SIP and carried over the Real Time Transport
   Protocol (RTP) [14]. The microphone input can be carried using any
   suitable voice encoding algorithm. The keypad input can be conveyed
   in one of two ways. The first is to convert the keypad input to DTMF,
   and then convey that DTMF using a suitance encoding algorithm for it
   (such as PCMU). An alternative, and generally the preferred approach,
   is to transmit the keypad input using RFC 2833 [15], which provides
   an encoding mechanism for carrying keypad input within RTP.

   In this classic model, the user interface would run on a server in
   the IP network. It would perform speech recognition and DTMF
   recognition to derive the user intent, feed them through the user
   interface, and provide the result to an application.

3.5.2 Client Local

   An alternative model is for the entire user interface to reside on



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   the telephone. The user interface can be a VoiceXML browser, running
   speech recognition on the microphone input, and feeding the keypad
   input directly into the script. As discussed above, the VoiceXML
   script could be rendered using text instead of voice, if the
   telephone had a textual display.

3.5.3 Flip-Flop

   A middle-ground approach is to flip back and forth between a
   client-local and client-remote user interface. Many voice
   applications are of the type which listen to the media stream and
   wait for some specific trigger that kicks off a more complex user
   interaction. The long pound in a pre-paid calling card application is
   one example. Another example is a conference recording application,
   where the user can press a key at some point in the call to begin
   recording. When the key is pressed, the user hears a whisper to
   inform them that recording has started.

   The ideal way to support such an application is to install a
   client-local user interface component that waits for the trigger to
   kick off the real interaction. Once the trigger is received, the
   application connects the user to a client-remote user interface that
   can play announements, collect more information, and so on.

   The benefit of flip-flopping between a client-local and client-remote
   user interface is cost. The client-local user interface will
   eliminate the need to send media streams into the network just to
   wait for the user to press the pound key on the keypad.

   The Keypad Markup Language (KPML) was designed to support exactly
   this kind of need [6]. It models the keypad on a phone, and allows an
   application to be informed when any sequence of keys have been
   pressed. However, KPML has no presentation component. Since user
   interfaces generally require a response to user input, the
   presentation will need to be done using a client-remote user
   interface that gets instantiated as a result of the trigger.

   It is tempting to use a hybrid model, where a prompt-and-collect
   application is implemented by using a client-remote user interface
   that plays the prompts, and a client-local user interface, described
   by KPML, that collects digits. However, this only complicates the
   application. Firstly, the keypad input will be sent to both the media
   stream and the KPML user interface. This requires the application to
   sort out which user inputs are duplicates, a process that is very
   complicated. Secondly, the primary benefit of KPML is to avoid having
   a media stream towards a user interface. However, there is already a
   media stream for the prompting, so there is no real savings.




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4. Framework Overview

   In this framework, we use the term "SIP application" to refer to a
   broad set of functionality. A SIP application is a program running on
   a SIP-based element (such as a proxy or user agent) that provides
   some value-added function to a user or system administrator. SIP
   applications can execute on behalf of a caller, a called party, or a
   multitude of users at once.

   Each application has a number of instances that are executing at any
   given time. An instance represents a single execution path for an
   application. Each instance has a well defined lifecycle. It is
   established as a result of some event. That event can be a SIP event,
   such as the reception of a SIP INVITE request, or it can be a non-SIP
   event, such as a web form post or even a timer. Application instances
   also have a specific end time. Some instances have a lifetime that is
   coupled with a SIP transaction or dialog. For example, a proxy
   application might begin when an INVITE arrives, and terminate when
   the call is answered. Other applications have a lifetime that spans
   multiple dialogs or transactions. For example, a conferencing
   application instance may exist so long as there are any dialogs
   connected to it. When the last dialog terminates, the application
   instance terminates. Other applications have a liftime that is
   completely decoupled from SIP events.

   It is fundamental to the framework described here that multiple
   application instances may interact with a user during a single SIP
   transaction or dialog. Each instance may be for the same application,
   or different applications. Each of the applications may be completely
   independent, in that they may be owned by different providers, and
   may not be aware of each others existence. Similarly, there may be
   application instances interacting with the caller, and instances
   interacting with the callee, both within the same transaction or
   dialog.

   The first step in the interaction with the user is to instantiate one
   of more user interface components for the application instance. A
   user interface component is a single piece of the user interface that
   is defined by a logical flow that is not synchronously coupled with
   any other component. In other words, each component runs more or less
   independently.

   A user interface component can be instantiated in one of the user
   agents in a dialog (for a client-local user interface), or within a
   network element (for a client-remote user interface). If a
   client-local user interface is to be used, the application needs to
   determine whether or not the user agent is capable of supporting a
   client-local user interface, and in what format. In this framework,



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   all client-local user interface components are described by a markup
   language. A markup language describes a logical flow of presentation
   of information to the user, collection of information from the user,
   and transmission of that information to an application. Examples of
   markup languages include HTML, WML, VoiceXML, and the Keypad Markup
   Language (KPML) [6].

   Unlike an application instance, which has very flexible lifetimes, a
   user interface component has a very fixed lifetime. A user interface
   component is always associated with a dialog. The user interface
   component can be created at any point after the dialog (or early
   dialog) is created. However, the user interface component terminates
   when the dialog terminates. The user interface component can be
   terminated earlier by the user agent, and possibly by the
   application, but its lifetime never exceeds that of its associated
   dialog.

   There are two ways to create a client local interface component. For
   interface components that are presentation capable, the application
   sends a REFER [5] request to the user agent. The Refer-To header
   field contains an HTTP URI that points to the markup for the user
   interface. For interface components that are presentation free (such
   as those defined by KPML), the application sends a SUBSCRIBE request
   to the user agent. The body of the SUBSCRIBE request contains a
   filter, which, in this case, is the markup that defines when
   information is to be sent to the application in a NOTIFY.

   If a user interface component is to be instantiated in the network,
   there is no need to determine the capabilities of the device on which
   the user interface is instantiated. Presumably, it is on a device on
   which the application knows a UI can be created. However, the
   application does need to connect the user device to the user
   interface. This will require manipulation of media streams in order
   to establish that connection.

   The interface between the user interface component and the
   application depends on the type of user interface. For presentation
   capable user interfaces, such as those described by  HTML and
   VoiceXML, HTTP form POST operations are used. For presentation free
   user interfaces, a SIP NOTIFY is used. The differing needs and
   capabilities of these two user interfaces, as described in Section
   3.4, is what drives the different choices for the interactions. Since
   presentation capable user interfaces require an update to the
   presentation every time user data is entered, they are a good match
   for HTTP. Since presentation free user interfaces merely transmit
   user input to the application, a NOTIFY is more appropriate.

   Indeed, for presentation free user interfaces, there are two



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   different modalities of operation. The first is called "one shot". In
   the one-shot role, the markup waits for a user to enter some
   information, and when they do, reports this event to the application.
   The application then does something, and the markup is no longer
   used. In the other modality, called "monitor", the markup stays
   permanently resident, and reports information back to an application
   until termination of the associated dialog.












































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5. Application Behavior

   The behavior of an application within this framework depends on
   whether it seeks to use a client-local or client-remote user
   interface.

5.1 Client Local Interfaces

   One key component of this framework is support for client local user
   interfaces.

5.1.1 Discovering Capabilities

   A client local user interface can only be instantiated on a user
   agent if the user agent supports that type of user interface
   component. Support for client local user interface components is
   declared by both the UAC and a UAS in its Accept, Allow, Contact and
   Allow-Event header fields of dialog-initiating requests and
   responses. If the Allow header field indicates support for the SIP
   SUBSCRIBE method, and the Allow-Event header field indicates support
   for the kpml package [6], and the Contact header field indicates that
   its URI is a GRUU [9] it means that the UA can instantiate
   presentation free user interface components. In this case, the
   application MAY push presentation free user interface components
   according to the rules of Section 5.1.2. The specific markup
   languages that can be supported are indicated in the Accept header
   field.

   If the Allow header field indicates support for the SIP REFER method,
   and the Contact header field contains UA capabilities [4] that
   indicate support for the HTTP URI scheme, it means that the UA
   supports presentation capable user interface components. In this
   case, the application MAY push presentation capable user interface
   components to the client according to the rules of Section 5.1.2. The
   specific markups that are supported are indicated in the Accept
   header field.

5.1.2 Pushing an Initial Interface Component

   Generally, we anticipate that interface components will need to be
   created at various different points in a SIP session. Clearly, they
   will need to be pushed during session setup, or after the session is
   established. A user interface component is always associated with a
   specific dialog, however.

   An application MUST NOT attempt to push a user interface component to
   a user agent until it has determined that the user agent has the
   neccesary capabilities and a dialog has been created. In the case of



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   a UAC, this means that an application MUST NOT push a user interface
   component for an INVITE initiated dialog until the application has
   seen a 200 OK followed by an ACK. For SUBSCRIBE initiated dialogs, it
   MUST NOT push a user interface component until the application has
   seen a 200 OK to the NOTIFY request. For a user interface component
   on a UAS, the application MUST NOT push a user interface component
   for an INVITE initiated dialog until it has seen a 200 OK from the
   UAS. For a SUBSCRIBE initiated dialog, it MUST NOT push a user
   interface component until it has seen a NOTIFY request from the
   notifier.

   To create a presentation capable UI component on the UA, the
   application sends a REFER request to the UA. This REFER MUST be sent
   to the Globally Routable UA URI (GRUU) [9] advertised by that UA in
   the Contact header field of the dialog initiating request or response
   sent by that UA.  Note that this REFER request creates a separate
   dialog between the application and the UA. The Refer-To header field
   of the REFER request MUST contain an HTTP URI that references the
   markup document to be fetched.

      OPEN ISSUE: The refer needs to provide a context to the UA, and in
      particular, identify the specific dialog that this component is
      associated with. There is no obvious candidate for this when REFER
      is used. The former proposal, of using a grid, cannot work because
      of forking.

   To create a presentation free user interface component, the
   application sends a SUBSCRIBE request to the UA. The SUBSCRIBE MUST
   be sent to the GRUU advertised by the UA. This SUBSCRIBE request
   creates a separate dialog. The SUBSCRIBE request MUST use the KPML
   [6] event package. The Event header field MUST contain parameters
   which identify the particular dialog that the interface component is
   being instantiated against. The body of the SUBSCRIBE request
   contains the markup document that defines the conditions under which
   the application wishes to be notified of user input.

   In both cases, the REFER or SUBSCRIBE request SHOULD include a
   display name in the From header field which identifies the name of
   the application. For example, a prepaid calling card might include a
   From header field which looks like:


   From: "Prepaid Calling Card" <sip:prepaid@example.com>

   To authenticate themselves, it is RECOMMENDED that applications use
   the SIP identity mechanism [7] in the REFER or SUBSCRIBE requests
   they generate. This mechanism has the benefit that the signature is
   over an authenticated identity body [8], which includes the From



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   header field. As such, the client can obtain cryptographic assurances
   about the service provider (the domain in the From header field)
   along with the name of the application.

5.1.3 Updating an Interface Component

   Once a user interface component has been created on a client, it can
   be updated. The means for updating it depends on the type of UI
   component.

   Presentation capable UI components are updated using techniques
   already in place for those markups. In particular, user input will
   cause an HTTP POST operation to push the user input to the
   application. The result of the POST operation is a new markup that
   the UI is supposed to use. This allows the UI to updated in response
   to user action. Some markups, such as HTML, provide the ability to
   force a refresh after a certain period of time, so that the UI can be
   updated without user input. Those mechanisms can be used here as
   well. However, there is no support for an asynchronous push of an
   updated UI component from the appliciation to the user agent. A new
   REFER request to the same GRUU would create a new UI component rather
   than updating any components already in place.

   For presentation free UI, the story is different. The application MAY
   update the filter at any time by generating a SUBSCRIBE refresh with
   the new filter. The UA will immediately begin using this new filter.

5.1.4 Terminating an Interface Component

   User interface components have a well defined lifetime. They are
   created when the component is first pushed to the client. User
   interface components are always associated with the SIP dialog on
   which they were pushed. As such, their lifetime is bound by the
   lifetime of the dialog. When the dialog ends, so does the interface
   component.

   However, there are some cases where the application would like to
   terminate the user interface component before its natural termination
   point. For presentation capable user interfaces, this is not
   possible. For presentation free user interfaces, the application MAY
   terminate the component by sending a SUBSCRIBE with Expires equal to
   zero. This terminates the subscription, which removes the UI
   component.

   A client can remove a UI component at any time. For presentation
   aware UI, this is analagous to the user dismissing the web form
   window. There is no mechanism provided for reporting this kind of
   event to the application. The applicatio MUST be prepared to time



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   out, and never receive input from a user. For presentation free user
   interfaces, the UA can explicitly terminate the subscription. This
   will result in the generation of a NOTIFY with a Subscription-State
   header field equal to "terminated".

5.2 Client Remote Interfaces

   As an alternative to, or in conjunction with client local user
   interfaces, an application can make use of client remote user
   interfaces. These user interfaces can execute co-resident with the
   application itself (in which case no standardized interfaces between
   the UI and the application need to be used), or it can run
   separately. This framework assumes that the user interface runs on a
   host that has a sufficient trust relationship with the application.
   As such, the means for instantiating the user interface is not
   considered here.

   The primary issue is to connect the user device to the remote user
   interface. Doing so requires the manipulation of media streams
   between the client and the user interface. Such manipulation can only
   be done by user agents. There are two types of user agent
   applications within this framework - originating/terminating
   applications, and intermediary applications.

5.2.1 Originating and Terminating Applications

   Originating and terminating applications are applications which are
   themselves the originator or the final recipient of a SIP invitation.
   They are "pure" user agent applications - not back-to-back user
   agents. The classic example of such an application is an interactive
   voice response (IVR) application, which is typically a terminating
   application. Its a terminating application because the user
   explicitly calls it; i.e., it is the actual called party. An example
   of an originating application is a wakeup call application, which
   calls a user at a specified time in order to wake them up.

   Because originating and terminating applications are a natural
   termination point of the dialog, manipulation of the media session by
   the application is trivial. Traditional SIP techniques for adding and
   removing media streams, modifying codecs, and changing the address of
   the recipient of the media streams, can be applied. Similarly, the
   application can directly authenticate itself to the user through S/
   MIME, since it is the peer UA in the dialog.

5.2.2 Intermediary Applications

   Intermediary applications are, at the same time, more common than
   originating/terminating applications, and more complex. Intermediary



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   applications are applications that are neither the actual caller or
   called party. Rather, they represent a "third party" that wishes to
   interact with the user. The classic example is the ubiquitous
   pre-paid calling card application.

   In order for the intermediary application to add a client remote user
   interface, it needs to manipulate the media streams of the user agent
   to terminate on that user interface. This also introduces a
   fundamental feature interaction issue. Since the intermediary
   application is not an actual participant in the call, how does the
   user interact with the intermediary application, and its actual peer
   in the dialog, at the same time? This is discussed in more detail in
   Section 7.






































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6. User Agent Behavior

6.1 Advertising Capabilities

   In order to participate in applications that make use of stimulus
   interfaces, a user agent needs to advertise its interaction
   capabilities.

   If a user agent supports presentation capable user interfaces, it
   MUST support the REFER method. It MUST include, in all dialog
   initiating requests and responses, an Allow header field that
   includes the REFER method. Furthermore, the UA MUST support the SIP
   user agent capabilities specification [4]. The UA MUST be capable of
   being REFER'd to an HTTP URI. It MUST include, in the Contact header
   field of its dialog initiating requests and responses, a "schemes"
   Contact header field parameter include the http URI scheme. The UA
   MUST include, in all dialog initiating requests and responses, an
   Accept header field listing all of those markups supported by the UA.
   It is RECOMMENDED that all user agents that support presentation
   capable user interfaces support HTML.

   If a user agent supports presentation free user interfaces, it MUST
   support the SUBSCRIBE [2] method. It MUST support the KPML [6] event
   package. It MUST include, in all dialog initiating requests and
   responses, an Allow header field that includes the SUBSCRIBE method.
   It MUST include, in all dialog initiating requests and responses, an
   Allow-Events header field that lists the KPML event package. The UA
   MUST include, in all dialog initiating requests and responses, an
   Accept header field listing those event filters it supports. At a
   minimum, a UA MUST support the "application/kpml+xml" MIME type.

   For either presentation free or presentation capable user interfaces,
   the user agent MUST support the GRUU [9] specification. The Contact
   header field in all dialog initiating requests and responses MUST
   contain a GRUU. The UA MUST include a Supported header field which
   contains the gruu option tag.

   Because these headers are examined by proxies which may be executing
   applications, a UA that wishes to support client local user
   interfaces should not encrypt them.

6.2 Receiving User Interface Components

   Once the UA has created a dialog (in either the early or confirmed
   states), it MUST be prepared to receive a SUBSCRIBE or REFER request
   against its GRUU. If the UA receives such a request prior to the
   establishment of a dialog, the UA MUST reject the request.




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   A user agent SHOULD attempt to authenticate the sender of the
   request. The sender will generally be an application, and therefore
   the user agent is unlikely to ever have a shared secret with it,
   making digest authentication useless. However, the REFER or SUBSCRIBE
   request should have a SIP authenticated identity body [8] that
   conveys the identity of the application [7]. If such a body is not
   present, and no alternative means of identification (such as
   P-Asserted-ID [11]) is present, the user agent MAY reject the request
   with a 403 response.

   Next, the user agent authorizes the application. An application is
   authorized to instantiate a user interface component if the
   application was resident within an element on the path of the dialog
   initiating request. An application proves to the user agent that it
   was on the path by presenting it with the dialog identifiers in the
   SUBSCRIBE or REFER request. In the case of SUBSCRIBE, those
   identifiers are present in the Event header field [6]. [[EDITORS
   NOTE: Fill in here once we know how this is done for REFER.]]

   Because of the dialog identifiers serve as a tool for authorization,
   a user agent compliant to this framework MUST use dialog identifiers
   that are cryptographically random, with at least 128 bits of
   randomness. It is recommended that this randomness be split between
   the Call-ID and From header field tag in the case of a UAC.

   Furthermore, to ensure that only applications resident in on-path
   elements can instantiate a user interface component, a user agent
   compliant to this specification SHOULD use the sips URI scheme for
   all dialogs it initiates. This will guarantee secure links between
   all of the elements on the signaling path.

   If an application does not present a valid dialog identifier in its
   REFER or SUBSCRIBE request, the user agent MUST reject the request
   with a 403 response. A user agent MAY apply any other policies in
   addition to (but not instead of) the ones specified here in order to
   authorize the creation of the user interface component. One such
   mechanism would be to prompt the user, informing them of the identity
   of the application. If an authorization policy requires user
   interaction, the user agent SHOULD respond to the SUBSCRIBE or REFER
   request with a 202. In the case of SUBSCRIBE, if authorization is not
   granted, the user agent SHOULD generate a NOTIFY to terminate the
   subscription. In the case of REFER, the user agent MUST NOT act upon
   the URI in the Refer-To header field until user authorization was
   obtained.

   If a REFER request to an HTTP URI was authorized, the UA executes the
   URI and fetches the content to be rendered to the user. This
   instantiates a presentation capable user interface component. If a



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   SUBSCRIBE was authorized, a presentation free user interface
   component was instantiated.

6.3 Mapping User Input to User Interface Components

   Once the user interface components are instantiated, the user agent
   must direct user input to the appropriate component. In the case of
   presentation capable user interfaces, this process is known as focus
   selection. It is done by means that are specific to the user
   interface on the device. In the case of a PC, for example, the window
   manager would allow the user to select the appropriate user interface
   component that their input is directed to.

   For presentation free user interfaces, the situation is more
   complicated. In some cases, the device may support a mechanism that
   allows the user to select a "line", and thus the associated dialog.
   Any user input on the keypad while this line is selected are fed to
   the user interface components associated with that dialog.

      TODO: Need to consider the case where the user interface is
      co-resident with the UAC, but the user device is separated from
      the UAC, and occurs through some other protocol, and the user
      interface and application are semi-trusted. Classic case is when
      the UAC is a PSTN gateway.


6.4 Receiving Updates to User Interface Components

   For presentation capable user interfaces, updates to the user
   interface occur in ways specific to that user interface component. In
   the case of HTML, for example, the document can tell the client to
   fetch a new document periodically. However, this framework does not
   provide any additional machinery to asynchronously push a new user
   interface component to the client.

   For presentation free user interfaces, an application can push an
   update to a component by sending a SUBSCRIBE refresh with a new
   filter. The user agent will process these according to the rules of
   the event package.

6.5 Terminating a User Interface Component

   Termination of a presentation capable user interface component is a
   trivial procedure. The user agent merely dismisses the window (or
   equivalent). The fact that the component is dismissed is not
   communicated to the application. As such, it is purely a local
   matter.




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   In the case of a presentation free user interface, if the user wishes
   to cease interacting with the application, it SHOULD generate a
   NOTIFY request with a Subscription-State equal to "terminated" and a
   reason of "rejected". This tells the application that the component
   has been removed, and that it should not attempt to re-subscribe.














































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7. Inter-Application Feature Interaction

   The inter-application feature interaction problem is inherent to
   stimulus signaling. Whenever there are multiple applications, there
   are multiple user interfaces. When the user provides an input, to
   which user interface is the input destined? That question is the
   essence of the inter-application feature interaction problem.

   Inter-application feature interaction is not an easy problem to
   resolve. For now, we consider separately the issues for client-local
   and client-remote user interface components.

7.1 Client Local UI

   When the user interface itself resides locally on the client device,
   the feature interaction problem is actually much simpler. The end
   device knows explicitly about each application, and therefore can
   present the user with each one separately. When the user provides
   input, the client device can determine to which user interface the
   input is destined. The user interface to which input is destined is
   referred to as the application in focus, and the means by which the
   focused application is selected is called focus determination.

   Generally speaking, focus determination is purely a local operation.
   In the PC universe, focus determination is provided by window
   managers. Each application does not know about focus, it merely
   receives the user input that has been targeted to it when its in
   focus. This basic concept applies to SIP-based applications as well.

   Focus determination will frequently be trivial, depending on the user
   interface type. Consider a user that makes a call from a PC. The call
   passes through a pre-paid calling card application, and a call
   recording application. Both of these wish to interact with the user.
   Both push an HTML-based user interface to the user. On the PC, each
   user interface would appear as a separate window. The user interacts
   with the call recording application by selecting its window, and with
   the pre-paid calling card application by selecting its window. Focus
   determination is literally provided by the PC window manager. It is
   clear to which application the user input is targeted.

   As another example, consider the same two applications, but on a
   "smart phone" that has a set of buttons, and next to each button, an
   LCD display that can provide the user with an option. This user
   interface can be represented using the Wireless Markup Language
   (WML).

   The phone would allocate some number of buttons to each application.
   The prepaid calling card would get one button for its "hangup"



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   command, and the recording application would get one for its "start/
   stop" command. The user can easily determine which application to
   interact with by pressing the appropriate button. Pressing a button
   determines focus and provides user input, both at the same time.

   Unfortunately, not all devices will have these advanced displays. A
   PSTN gateway, or a basic IP telephone, may only have a 12-key keypad.
   The user interfaces for these devices are provided through the Keypad
   Markup Language (KPML). Considering once again the feature
   interaction case above, the pre-paid calling card application and the
   call recording application would both pass a KPML document to the
   device. When the user presses a button on the keypad, to which
   document does the input apply? The user interface does not allow the
   user to select. A user interface where the user cannot provide focus
   is called a focusless user interface. This is quite a hard problem to
   solve. This framework does not make any explicit normative
   recommendation, but concludes that the best option is to send the
   input to both user interfaces unless the markup in one interface has
   indicated that it should be suppressed from others. This is a
   sensible choice by analogy - its exactly what the existing circuit
   switched telephone network will do. It is an explicit non-goal to
   provide a better mechanism for feature interaction resolution than
   the PSTN on devices which have the same user interface as they do on
   the PSTN. Devices with better displays, such as PCs or screen phones,
   can benefit from the capabilities of this framework, allowing the
   user to determine which application they are interacting with.

   Indeed, when a user provides input on a focusless device, the input
   must be passed to all client local user interfaces, AND all client
   remote user interfaces, unless the markup tells the UI to suppress
   the media. In the case of KPML, key events are passed to remote user
   interfaces by encoding them in RFC 2833 [15]. Of course, since a
   client cannot determine if a media stream terminates in a remote user
   interface or not, these key events are passed in all audio media
   streams unless the "Q" digit is used to suppress.

7.2 Client-Remote UI

   When the user interfaces run remotely, the determination of focus can
   be much, much harder. There are many architectures that can be
   deployed to handle the interaction. None are ideal. However, all are
   beyond the scope of this specification.









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8. Intra Application Feature Interaction

   An application can instantiate a multiplicity of user interface
   components. For example, a single application can instantiate two
   separate HTML components and one WML component. Furthermore, an
   application can instantiate both client local and client remote user
   interfaces.

   The feature interaction issues between these components within the
   same application are less severe. If an application has multiple
   client user interface components, their interaction is resolved
   identically to the inter-application case - through focus
   determination. However, the problems in focusless user interfaces
   (such as a keypad) generally won't exist, since the application can
   generate user interfaces which do not overlap in their usage of an
   input.

   The real issue is that the optimal user experience frequently
   requires some kind of coupling between the differing user interface
   components. This is a classic problem in multi-modal user interfaces,
   such as those described by Speech Application Language Tags (SALT).
   As an example, consider a user interface where a user can either
   press a labeled button to make a selection, or listen to a prompt,
   and speak the desired selection. Ideally, when the user presses the
   button, the prompt should cease immediately, since both of them were
   targeted at collecting the same information in parallel. Such
   interactions are best handled by markups which natively support such
   interactions, such as SALT, and thus require no explicit support from
   this framework.






















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9. Example Call Flow

   This section shows the operation of a call recording application.
   This application allows a user to record the media in their call by
   clicking on a button in a web form. The application uses a
   presentation capable user interface component that is pushed to the
   caller.


             A                  Recording App                  B
             |(1) INVITE              |                        |
             |----------------------->|                        |
             |                        |(2) INVITE              |
             |                        |----------------------->|
             |                        |(3) 200 OK              |
             |                        |<-----------------------|
             |(4) 200 OK              |                        |
             |<-----------------------|                        |
             |(5) ACK                 |                        |
             |----------------------->|                        |
             |                        |(6) ACK                 |
             |                        |----------------------->|
             |(7) REFER               |                        |
             |<-----------------------|                        |
             |(8) 200 OK              |                        |
             |----------------------->|                        |
             |(9) NOTIFY              |                        |
             |----------------------->|                        |
             |(10) 200 OK             |                        |
             |<-----------------------|                        |
             |(11) HTTP GET           |                        |
             |----------------------->|                        |
             |(12) 200 OK             |                        |
             |<-----------------------|                        |
             |(13) HTTP POST          |                        |
             |----------------------->|                        |
             |(14) 200 OK             |                        |
             |<-----------------------|                        |


                                Figure 3

   First, the caller, A, sends an INVITE to setup a call (message 1).
   Since the caller supports the framework, and can handle presentation
   capable user interface components, it includes the Supported header
   field indicating the GRUU is understood, Allow indicating that REFER
   is understood, and a Contact header field that includes the "schemes"
   header field parameter.



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   INVITE sip:B@example.com SIP/2.0
   Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9zz8
   From: Caller <sip:A@example.com>;tag=kkaz-
   To: Callee <sip:B@example.com>
   Call-ID: faif9a@host.example.com
   CSeq: 1 INVITE
   Supported: gruu
   Allow: INVITE, OPTIONS, BYE, CANCEL, ACK, REFER
   Contact: <sip:bad998asd8asd0000a@example.com>;schemes="http,sip"
   Content-Length: ...
   Content-Type: application/sdp

   --SDP not shown--

   The proxy acts as a recording server, and forwards the INVITE to the
   called party (message 2):


   INVITE sip:B@pc.example.com SIP/2.0
   Record-Route: <sip:app.example.com;lr>
   Via: SIP/2.0/UDP app.example.com;branch=z9hG4bK97sh
   Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9zz8
   From: Caller <sip:A@example.com>;tag=kkaz-
   To: Callee <sip:B@example.com>
   Call-ID: faif9a@host.example.com
   CSeq: 1 INVITE
   Supported: gruu
   Allow: INVITE, OPTIONS, BYE, CANCEL, ACK, REFER
   Contact: <sip:bad998asd8asd0000a@example.com>;schemes="http,sip"
   Content-Length: ...
   Content-Type: application/sdp

   --SDP not shown--

   B accepts the call with a 200 OK (message 3). It does not support the
   framework, and so the various header fields are not present.


   SIP/2.0 200 OK
   Record-Route: <sip:app.example.com;lr>
   Via: SIP/2.0/UDP app.example.com;branch=z9hG4bK97sh
   Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9zz8
   From: Caller <sip:A@example.com>;tag=kkaz-
   To: Callee <sip:B@example.com>;tag=7777
   Call-ID: faif9a@host.example.com
   CSeq: 1 INVITE
   Contact: <sip:B@pc.example.com>
   Content-Length: ...



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   Content-Type: application/sdp

   --SDP not shown--

   This 200 OK is passed back to the caller (message 4):


   SIP/2.0 200 OK
   Record-Route: <sip:app.example.com;lr>
   Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9zz8
   From: Caller <sip:A@example.com>;tag=kkaz-
   To: Callee <sip:B@example.com>;tag=7777
   Call-ID: faif9a@host.example.com
   CSeq: 1 INVITE
   Contact: <sip:B@pc.example.com>
   Content-Length: ...
   Content-Type: application/sdp

   --SDP not shown--

   The caller generates an ACK (message 5).


   ACK sip:B@pc.example.com
   Route: <sip:app.example.com;lr>
   Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9zz9
   From: Caller <sip:A@example.com>;tag=kkaz-
   To: Callee <sip:B@example.com>;tag=7777
   Call-ID: faif9a@host.example.com
   CSeq: 1 ACK

   The ACK is forwarded to the called party (message 6).


   ACK sip:B@pc.example.com
   Via: SIP/2.0/UDP app.example.com;branch=z9hG4bKh7s
   Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9zz9
   From: Caller <sip:A@example.com>;tag=kkaz-
   To: Callee <sip:B@example.com>;tag=7777
   Call-ID: faif9a@host.example.com
   CSeq: 1 ACK

   Now, the application decides to push a user interface component to
   user A. So, it sends it a REFER request (message 7):







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   REFER sip:bad998asd8asd0000a@example.com SIP/2.0
   Refer-To: http://app.example.com/script.pl
   Via: SIP/2.0/UDP app.example.com;branch=z9hG4bK9zh6
   From: Recorder Application <sip:app.example.com>;tag=jhgf
   To: Caller <sip:A@example.com>
   Call-ID: 66676776767@app.example.com
   CSeq: 1 REFER
   Event: refer
   Contact: <sip:sip:app.example.com>

   The REFER is answered by a 200 OK (message 8).


   SIP/2.0 200 OK
   Refer-To: http://app.example.com/script.pl
   Via: SIP/2.0/UDP app.example.com;branch=z9hG4bK9zh6
   From: Recorder Application <sip:app.example.com>;tag=jhgf
   To: Caller <sip:A@example.com>;tag=pqoew
   Call-ID: 66676776767@app.example.com
   Supported: gruu
   Allow: INVITE, OPTIONS, BYE, CANCEL, ACK, REFER
   Contact: <sip:bad998asd8asd0000a@example.com>;schemes="http,sip"
   CSeq: 1 REFER

   User A sends a NOTIFY (message 9):


   NOTIFY sip:app.example.com SIP/2.0
   Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9320394238995
   To: Recorder Application <sip:app.example.com>;tag=jhgf
   From: Caller <sip:A@example.com>;tag=pqoew
   Call-ID: 66676776767@app.example.com
   CSeq: 1 NOTIFY
   Max-Forwards: 70
   Event: refer;id=93809824
   Subscription-State: active;expires=3600
   Contact: <sip:bad998asd8asd0000a@example.com>;schemes="http,sip"
   Content-Type: message/sipfrag;version=2.0
   Content-Length: 20

   SIP/2.0 100 Trying

   And the recording server responds with a 200 OK (message 10)


   SIP/2.0 200 OK
   Via: SIP/2.0/UDP host.example.com;branch=z9hG4bK9320394238995
   To: Recorder Application <sip:app.example.com>;tag=jhgf



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   From: Caller <sip:A@example.com>;tag=pqoew
   Call-ID: 66676776767@app.example.com
   CSeq: 1 NOTIFY

   The caller, A, authorizes the application. It then acts on the
   Refer-To URI, fetching the script from app.example.com (message 11).
   The response, message 12, contains a web application that the user
   can click on to enable recording. When the user clicks on the link
   (message 13), the results are posted to the server, and an updated
   display is provided (message 14).









































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10. Security Considerations

   There are many security considerations associated with this
   framework. It allows applications in the network to instantiate user
   interface components on a client device. Such instantiations need to
   be from authenticated applications, and also need to be authorized to
   place a UI into the client. Indeed, the stronger requirement is
   authorization. It is not so important to know that name of the
   provider of the application, but rather, that the provider is
   authorized to instantiate components.

   Generally, an application should be considered authorized if it was
   an application that was legitimately part of the call setup path.
   With this definition, authorization can be enforced using the sips
   URI scheme when the call is initiated.




































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11. Contributors

   This document was produced as a result of discussions amongst the
   application interaction design team. All members of this team
   contributed significantly to the ideas embodied in this document. The
   members of this team were:


   Eric Burger
   Cullen Jennings
   Robert Fairlie-Cuninghame








































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Normative References

   [1]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
        Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
        Session Initiation Protocol", RFC 3261, June 2002.

   [2]  Roach, A., "Session Initiation Protocol (SIP)-Specific Event
        Notification", RFC 3265, June 2002.

   [3]  McGlashan, S., Lucas, B., Porter, B., Rehor, K., Burnett, D.,
        Carter, J., Ferrans, J. and A. Hunt, "Voice Extensible Markup
        Language (VoiceXML) Version 2.0", W3C CR CR-voicexml20-20030220,
        February 2003.

   [4]  Rosenberg, J., "Indicating User Agent Capabilities in the
        Session Initiation Protocol  (SIP)",
        draft-ietf-sip-callee-caps-03 (work in progress), January 2004.

   [5]  Sparks, R., "The Session Initiation Protocol (SIP) Refer
        Method", RFC 3515, April 2003.

   [6]  Burger, E., "Keypad Stimulus Protocol (KPML)",
        draft-ietf-sipping-kpml-02 (work in progress), February 2004.

   [7]  Peterson, J., "Enhancements for Authenticated Identity
        Management in the Session  Initiation Protocol (SIP)",
        draft-ietf-sip-identity-01 (work in progress), March 2003.

   [8]  Peterson, J., "SIP Authenticated Identity Body (AIB) Format",
        draft-ietf-sip-authid-body-02 (work in progress), July 2003.

   [9]  Rosenberg, J., "Obtaining and Using Globally Routable User Agent
        (UA) URIs (GRUU) in the  Session Initiation Protocol (SIP)",
        draft-ietf-sip-gruu-00 (work in progress), January 2004.

















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Informative References

   [10]  Day, M., Rosenberg, J. and H. Sugano, "A Model for Presence and
         Instant Messaging", RFC 2778, February 2000.

   [11]  Jennings, C., Peterson, J. and M. Watson, "Private Extensions
         to the Session Initiation Protocol (SIP) for Asserted Identity
         within Trusted Networks", RFC 3325, November 2002.

   [12]  Rosenberg, J., "A Framework for Conferencing with the Session
         Initiation Protocol",
         draft-ietf-sipping-conferencing-framework-01 (work in
         progress), October 2003.

   [13]  Rosenberg, J., Schulzrinne, H. and P. Kyzivat, "Caller
         Preferences for the Session Initiation Protocol (SIP)",
         draft-ietf-sip-callerprefs-10 (work in progress), October 2003.

   [14]  Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", RFC
         3550, July 2003.

   [15]  Schulzrinne, H. and S. Petrack, "RTP Payload for DTMF Digits,
         Telephony Tones and Telephony Signals", RFC 2833, May 2000.


Author's Address

   Jonathan Rosenberg
   dynamicsoft
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

   Phone: +1 973 952-5000
   EMail: jdrosen@dynamicsoft.com
   URI:   http://www.jdrosen.net














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Intellectual Property Statement

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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgment

   Funding for the RFC Editor function is currently provided by the
   Internet Society.











































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